The present invention relates to a method of monitoring transverse sealing on a packaging unit for continuously forming sealed packages containing pourable food products.
Many pourable food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of such a package is the parallelepiped packaging container for liquid or pourable food products known by the name of Tetra Brik Asepti(registered trademark), which is formed by folding and sealing a strip-rolled packaging material. The packaging material, a cross section of which is shown in FIG. 1 in which the material is indicated as a whole by 1, comprises a layer of fibrous material 2, e.g. paper, covered on both sides with two layers of thermoplastic material 3, 4, e.g. polyethylene; and, adjacent to the layer of thermoplastic material 3 facing inwards of the container, there is a barrier layer 5 of electrically conducting material, e.g. aluminium, in turn covered with a further layer of thermoplastic material 6, preferably also polyethylene.
As described for example in EP-A-O 460 540, such packages are produced on fully automatic packaging units, as shown and indicated by 10 in FIG. 2, on which the strip 1 of packaging material, fed off a reel 11, is sterilized (not shown), e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide or nitrogen peroxide solution; following sterilization, the sterilizing agent is removed, e.g. vaporized by heating, from the surfaces of the packaging material; and the strip 1 of packaging material so sterilized is kept in a closed sterile environment, and is folded and sealed longitudinally by means of rollers 9 to form a tube 12.
The tube 12 is filled with the sterilized or sterile-processed food product by means of a filling conduit 13, and is sealed and cut at equally spaced cross sections into pillow-pack packages, which are subsequently folded mechanically to form final packages 14, in the example shown of parallelepiped shape.
More specifically, sealing and cutting are performed by means of jaws 15 operating on one side of tube 12 and cooperating with respective counter-jaws not shown in FIG. 2. Jaws 15 and the respective counter-jaws move back and forth lengthwise of tube 12 (to move into position over the sealing and cutting region, and to follow the forward movement of tube 12 as it is being sealed and cut), and move to and from tube 12, as described in detail in EP-B-0 091 712. For which purpose, jaws 15 and the respective counter-jaws are controlled by rods 20 shown only schematically in FIG. 2 and controlled by cams not shown.
Each jaw 15 carries a respective inductor, as explained in detail later on, which is supplied by a high-frequency current source 21 to induce current in and so heat barrier layer 5 by the Joule effect. The facing layers of thermoplastic material 5 and 6 are thus softened, and simultaneous compression of tube 12 by the jaw/counter-jaw pair seals tube 12 transversely.
A central processing unit 22 controls and monitors the entire operation of packaging unit 10 to ensure the various operations are synchronized properly, to indicate any malfunctions, and to reject any packages 14 which cannot be guaranteed up to standard.
On units of the above type, sealing quality control is obviously of vital importance, both to ensure the production of perfectly sound packages, and to preserve the aseptic characteristics of the contents, on which the health of the consumer depends.
Consequently, a strong demand exists for some way of monitoring sealing to determine the quality of the seal and, in the course of the production process, determine any departure from the established standard quality level requiring corrective measures or rejection of the packages.
At present, quality control is based on determining the electric parameters (current, output voltage) and correct operation of the generator, and on manual inspection of the packages by the operator, which may consist solely in visual examination of the outside of the package, or also in tearing open the package to examine it from the inside and determine the tear resistance. Manual inspection, even when performed only externally, is obviously extremely slow with respect to the packaging speed of the unit, and, when performed internally, only allows of checking random samples. As a result, in the event of a faulty seal being detected, a large amount of product is wasted in the time interval between detecting the fault and arresting packaging unit 10. Moreover, the tedious nature of the job makes manual inspection fairly unreliable, due to possible distraction on the part of the operator.
On the other hand, checking the electric parameters alone is not enough, and only some of the parameters affecting sealing qualityxe2x80x94parameters relating to the characteristics of the material for sealing, the characteristics of the sterile product, operation of the sealing components, and sometimes random factors (one example which applies to all: the presence of product fibers or seeds in the sealing region)xe2x80x94are detectable directly by means of appropriate sensors. Moreover, the effects of some situations, not critical in themselves, at times combine unexpectedly to produce critical conditions (e.g. a slight increase in the thickness of the packaging material combined with a small reduction in current supply may result in a poor seal), so that detecting individual sealing parameters and comparing them with respective threshold values is not enough to detect all the critical situations that may arise. On the other hand, creating a mathematical model capable of taking into account all the effects and parameters involved would be impossible, on account of the complexity of the system.
To control transverse sealing quality, methods are currently being studied based on measuring the output power of the generator. Such methods, however, only provide for indicating a few malfunctions relative, for example, to faults on the current source itself, and, as the power supplied is never exploited fully, fail to provide for evaluating all the aspects affecting sealing quality.
A demand therefore exists for a reliable quality control method capable of rapidly indicating any critical situation that may arise.
It is an object amongst other objectives of the present invention to provide a quality control method which is more complete and more reliable as compared with known methods.
According to the present invention, there is provided a method of monitoring transverse sealing on a packaging unit for continuously forming sealed packages containing pourable food products, as claimed in claim 1.